1. Field of Invention
The invention relates to the provision of logic elements for logic devices and logic circuitry using the properties of magnetic quantum cellular automata, and in particular to an architecture for the design of such logic elements and devices and for the design and operation of logic circuitry using such elements.
2. Description of Related Art
The number of transistors on a microchip has doubled every 18 months for the past 30 years, a phenomenon known as ‘Moore's Law’. Fundamental physical phenomena associated with small particles make this exponential growth increasingly difficult to sustain. One option already being considered by the semiconductor memory industry is to incorporate nanometre sized magnetic particles (‘nanomagnets’) into microelectronic devices. A computer system requires not only memory, however, but also the ability to process information, the latter usually being achieved through electronic logic devices.
A new device architecture has been proposed constructed from nanomagnets which could be used to perform logic functions magnetically (Cowburn and Welland, Science Vol 287 pp 1466-1468). These so-called magnetic quantum cellular automata (MQCA) devices offer potentially 40,000 times higher integration density than conventional microelectronics while only dissipating a fraction of the power.
The magnetic logic system described in the reference described arrays of magnetic nanodots and uses magnetic solitons to carry information. A magnetic soliton is simply a semi-abrupt and mobile transition from one magnetisation direction to another. There are two important parameters associated with any component of such a magnetic logic system: its soliton propagation field and its soliton nucleation field. The soliton propagation field is the strength of magnetic field which must be applied to move the soliton freely through the logic component. It is a measure of the resistance to motion which the soliton experiences and is analogous to (static) friction in mechanical systems. The soliton nucleation field is the strength of magnetic field which must be applied to create spontaneously a pair of solitons in a logic component. Such spontaneous creation is undesirable as it leads to an erroneous logic state being generated. Soliton nucleation is analogous to electrical break-down in electronic systems.
An ideal magnetic logic device would have a propagation field very close to zero (solitons propagate completely freely without any resistance to motion) and a very high nucleation field (errors are very difficult to create). The figure of merit for any real magnetic logic component is the field separation between the soliton propagation field and nucleation field: it must be possible to apply a magnetic field which is strong enough to propagate existing solitons without erroneously nucleating new ones. The larger the gap between the propagation and nucleation fields, the greater the distribution in manufacturing tolerances that the device can tolerate.
To generate practical logic devices it is necessary to develop a practical architecture for the interconnects and logic gates which are required to make up the device. In the Cowburn and Welland reference solitons are propagated along a track in the device by coupling between the adjacent discrete magnetic regions comprised by magnetic nanodots. Whilst the field necessary to propagate solitons in a straight line is relatively small, changes in direction, junctions, and logic gates will result in poorer coupling between adjacent solitons. The resulting energy discontinuity necessitates higher propagation fields, reducing the gap between the propagation and the nucleation fields, and leading to the problems above described. The discontinuities were exacerbated by the discontinuous structure of the MQCAs in the reference paper.